Oliver has already spent an entire day in the lesser dimension, making him well-acquainted with its day-and-night cycle.
In the lesser dimension, there were 16 hours of daylight followed by 8 hours of night.
He had been diligent in tracking the passage of time ever since he arrived in this dimension, and he was certain that 5 hours of the night had already elapsed. In about three hours, the sun of the lesser dimension will swap places with its moon, and that'll signal the end of the hunting time for all the devils in it.
"Once dawn breaks, these fiends will be compelled to retreat to areas where the sun's shine can't extend to. All I need to do to stay safe is stick around in the cave until then."